The invention relates to a method for manufacturing a light emitting diode with a frosted surface.
Green luminous light-emitting diodes are made from (1 1 1)-oriented gallium phosphide. Since gallium phosphide is an indirect semiconductor, the efficiency is lower than in mixed crystal systems where, with a suitable composition, direct band-to-band transitions are also possible. The external quantum efficiency, which is determined by the material property of internal quantum efficiency and by the losses which occur when the radiation emerges from within the diode, is no more than 0.3% for GaP diodes.
One reason for the poor efficiency of the diodes is the high proportion of radiation that cannot escape from the body of the diode because of the total reflection on the surface of the semiconductor. This is due to the high optical refractive index of the semiconductor material. This is approximately 3.4 for gallium phosphide. This results in a critical angle of total reflection of 17.7.degree. on emergence of the radiation to air. By direct means, only that proportion of the radiation is emitted that falls on the boundary layer at a smaller angle to the surface normal. The remainder is reflected back into the diode body. A large part of the radiation reflected back is lost due to absorption in the semiconductor body and at the metal contacts. The efficiency can therefore be increased considerably if the emission of the radiation is improved.
In principle, there are many means of improving radiation emission from the interior. In the patent publication DE 42 31 007 A1, a method is described for increasing the critical angle of total reflection by applying a .lambda./4-thick antireflective coating.
From EP 404 565, a method for manufacturing a radiation-emitting diode made from the III-V compound-semiconductor material GaAIAs is known where the entire surface of the semiconductor chip is roughened in order to improve the external quantum efficiency. The roughening or frosting takes place after the diodes have been singled out. By frosting, the total reflection of the radiation generated at the boundary layer between the diode chip and the surrounding material is largely avoided, the light path in the semiconductor material is shortened and thus the probability of reabsorption is reduced. At the same time, the effective surface of the diode is increased in size thereby allowing more radiation to escape from the inside of the diode. A disadvantage of the known method is, however, that the diode chip is very difficult to bond at the time of assembly because of the etched contact layer structures. Also, etch-resistant materials must be used for the contact layer structures. Furthermore, etching of the surface in the region of the radiation-emitting junction results in a reduction of the life of the diodes.
From DE 43 05 296 A1, a method for manufacturing a radiation-emitting diode with frosted side edges is known. Frosting takes place after the substrate wafer has been diced. The diodes are held together in a group by a carrier sheet applied on the rear side. A layer of silicon dioxide protects the contact surfaces, the front side of the diode and the mesa edges with the pn junction that emerge at the surface against attack by etching. The silicon dioxide must be removed again after frosting. The application and removal of the protective layer and the bonding of the carrier sheet make the known method very elaborate.